This invention relates to wireless communication systems and, more particularly, to the transmission of data in such systems.
In wireless communication systems generally, base stations transmit signals to mobile terminals over a communication link referred to as a forward link, and mobile terminals transmit signals to base station over a communication link referred to as a reverse link. When a callxe2x80x94a communication session between a base station and a particular mobile terminalxe2x80x94is set up, a primary channel is set up on both a forward and reverse link. The primary channel can be used to transmit voice, data, and/or so-called signaling information, and transmits the signal at a particular, typically a fairly low, transmission rate. If it is desired to transmit a signal at a higher transmission rate, for example for a data transmission having a large amount of data, the wireless communication system may be able to set up a secondary channelxe2x80x94a communication channel over which signals may be transmitted at the same or at a higher transmission rate than that of the primary channel. The secondary channel is typically set up only over the particular communication link, i.e. forward or reverse, over which it is desired to transmit the signal.
The way in which the secondary channel is set up takes into account the fact that data is often bursty, meaning that it is transmitted in bursts interspersed with periods of inactivity during which no data is transmitted. Typically, the secondary channel is maintained only for the duration of each data burst. Between data bursts, there is no secondary channel assigned to the call, whereas there is a primary channel maintained for the duration of the call.
The transmission rate of the burst on the secondary channel is a critical factor affecting the efficiency of the wireless communication system. An unnecessarily low transmission rate leads to an unnecessarily long time to transmit the data and an inefficient use of air bandwidth resources. On the other hand, a too high transmission rate can result in a such a large amount of system resources being allocated to the call as to compromise the system""s ability to service other calls, and in so-called sectors where such a large amount of resources are allocated it can cause the equipment servicing the sector to enter into overload.
Usually, the main factor that limits how high a transmission rate can be used on the secondary channel is the power needed to transmit the signal, and in particular the initial power level on the secondary channel. Thus, the initial power level on the secondary channel is a critical factor affecting system efficiency. A too low initial power level results in an unacceptable level of received signal quality, which may cause so-called link errors. On the other hand, a too high initial power level leaves little transmit power for the system to service other calls, which degrades the overall data throughput and efficiency of the communication link. Another disadvantage of a too high initial power level is that the signal""s interference with calls involving other mobile terminals is increased unnecessarily, requiring that the power level of the signals on these calls be increased. At best, this further reduces the power available to service other mobile terminals and reduces the access furnished to new calls. At worst, if there is not enough power available to increase the power level of the signals on the other calls, one or more of those other calls may have to be dropped.
Advantageously, the wireless communication system""s power control will eventually adjust the power level to produce an efficient power level that will result in an acceptable level of received signal quality without causing unnecessarily strong interference with calls involving other mobile terminals. However, during the time it takes power control to accomplish this, if the initial power level was too high, the signal will cause unnecessarily strong interference with other calls and leave little transmit power for the system to service other calls. If the initial power level was too low, then, during the time it takes the power control to adjust it, the signal will be received with an unacceptable level of received signal quality. Additionally, if there is a large difference between the initial power level and the efficient power level, then the power control may not even be able to adjust the power level to the efficient power level before the burst is over.
Therefore, it is desirable for the wireless communication system to accurately estimate the power level it will allocate to the burst initially. Typically, the wireless communication system will base the initial power level on the power level of a signal on the primary channel just before the start of the burst, as described, for example, in U.S. patent application Ser. No. 09/676,179 entitled xe2x80x9cForward Transmission rate Determination of High Data Transmission Rate Channels in CDMA Air Interface,xe2x80x9d assigned to the present assignee and hereby incorporated by reference.
Once the initial power level is accurately estimated, it can be used to determine an efficient transmission rate, which is typically the highest transmission rate supportable by the available system resources.
The present inventors have recognized that the above-mentioned technique for determining the initial power level, and therefore the efficient transmission rate, of a burst may not provide accurate results under certain circumstances, as will now be described. During at least a portion of a call, a mobile terminal may be involved in a so-called soft handoff, in which it is communicating with more than one base station. The communication links between the mobile terminal and a particular base station are each referred to as a xe2x80x9clegxe2x80x9d of the handoff. When a secondary channel is established, it can be established on all the legs of the handoff. However, having the secondary channel on more than one leg of the handoff requires a significant amount of system resources and design complexity. Therefore, when conditions permit, it is known to establish the secondary channel on fewer than all the legs of the handoff. In such a case however, the communication-link characteristics of the primary channel of a particular leg are no longer similar to the communication-link characteristics of the secondary channel. This is caused by many factors including the fact that the communication-link characteristics on each of the legs of the handoff are both different and rapidly changing, and that there is no longer so-called space diversity on the secondary channel. We have thus recognized that the power level of a signal on the primary channel of a particular leg just before the start of the burst on the secondary channel is not necessarily an accurate indication of an appropriate initial power level for the burst on the secondary channel.
The present invention is a technique that allows for a more efficient initial power level, and therefore a more efficient transmission rate, for a secondary channel communication, for example a burst, when the secondary channel is on fewer legs of a handoff, such as a soft handoff, than the primary channel. In accordance with the invention, the initial power level of the burst transmitted over a current secondary channel on a communication link that includes a primary channel and a previous secondary channel is a function of a power level on the previous secondary channel, i.e. of a previous burst transmitted over the previous secondary channel. For example, the initial power level of the burst can be based on the power level prior to, or at, the termination of the previous burst, where the termination of the previous burst was within a predetermined time interval of the start of the burst. Preferably, the current secondary channel is on the identical legs of the handoff as the previous secondary channel. Optionally, the initial power level of a burst can also be a function of the characteristics of the communication link of the primary channel, the previous secondary channel, and the current secondary channel.
In accordance with a feature of the invention, the transmission rate may be adjusted based on the initial power level determined as described above to obtain a more efficient transmission rate. Particularly, the transmission rate may be adjusted based on whether the initial power level of the burst is acceptable in light of the power available at the power amplifier. In particular, the transmission rate can first be determined based on the system resources other than power. The initial power level is then determined using the above method with that transmission rate. If the initial power level is greater than the power available at the power amplifier, a lower transmission rate is selected and the initial power level is determined again using the above method but at the lower transmission rate.